1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) and a method of manufacturing the same. More particularly, the present invention relates to an MRAM whose memory cells respectively include two magnetic layers separated by a tunneling barrier layer, and a method of manufacturing the same.
2. Description of the Related Art
An MRAM, which integrates ferromagnetic layers to store digital data, is one of the promising nonvolatile memories. The MRAM stores digital data as directions of spontaneous magnetizations of the ferromagnetic layers. The directions of the spontaneous magnetizations are not reversed until an external magnetic field is applied to the ferromagnetic layers, and this achieves nonvolatile storage of the digital data in the MRAM.
To improve operation and structure of MRAMs, use of the tunnel magnetroresistance (TMR) effect has been proposed. The memory cell whose operation is based on the TMR effect includes two ferromagnetic layers separated by an insulating layer. The insulating layer is so thin that a tunneling current is allowed to pass though the insulating layer. The insulating layer typically has a thickness of about 1.5 nm. The TMR effect causes the resistance of the insulating layer to be changed depending on whether the spontaneous magnetizations of the two magnetic films are xe2x80x9cparallelxe2x80x9d or xe2x80x9cantiparallelxe2x80x9d. The change in the resistance allows the detection of the data stored in the memory cells.
The method of manufacturing the MRAM based on the TMR effect is disclosed in Japanese Laid Open Patent Application (JP-A 2000-353791). FIGS. 1A, 1B and 1C schematically show the conventional method of manufacturing the MRAM. As shown in FIG. 1A, a silicon oxide film 102, an aluminum film 103, a first magnetic film 104, an insulating film 105 and a second magnetic film 106 are formed in series on a substrate 101. A thickness of the insulating film 105 is so thin that a tunneling current passes through the insulating film 105.
After forming a photoresist 107 on the second magnetic film 106, as shown in FIG. 1B, the second magnetic film 106, the insulating film 105 and the first magnetic film 104 are etched with the photoresist 107 used as a mask. The etching fabricates a lower magnetic layer 104xe2x80x2, a tunneling barrier layer 105xe2x80x2 and an upper magnetic layer 106xe2x80x2. The lower magnetic layer 104xe2x80x2, the tunneling barrier layer 105xe2x80x2 and the upper magnetic layer 106xe2x80x2 constitute a memory cell. After the formation of the memory cell, as shown in FIG. 1C, the aluminum film 103 is etched to form a lower electrode 103xe2x80x2.
The conventional method causes mechanic stress to be applied to the insulating film 105 and the mechanical stress induces defects in the tunneling barrier layer 105xe2x80x2. The mechanical stress is generated in various ways in the process for manufacturing the MRAM. For example, the fixation of the substrate 101 to a manufacturing apparatus causes mechanical stress to be applied to the tunneling barrier layer 105xe2x80x2. Moreover, thermally-induced mechanical stress is applied to the insulating film 105 because of the difference between thermal expansion coefficients of the substrate 101, the silicon oxide film 102, the lower electrode 103, the first magnetic film 104, the second magnetic film 106 and the insulating film 105. The mechanical stress induces defects in the insulating film 105 and the induced defects may cause operational errors of the MRAM and thus degrade the reliability of the MRAM.
The stress-induced defects are desirably excluded from the tunneling barrier layer in the memory cell.
Another method of manufacturing an MRAM is disclosed in U.S. Pat. No. 6,153,443. In the other conventional method, a tunnel insulating film is discontinuously deposited between two magnetic films.
Furthermore, a method of manufacturing a thin film magnet head, which may be related to the present invention, is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 7-235016). In the document, it is disclosed that a curved insulating film is formed between two magnet films.
Therefore, an object of the present invention is to provide a magnetic random access memory for excluding stress-induced defects in a tunneling barrier layer included in a memory cell, and a method of manufacturing the same.
Another object of the present invention is to provide a magnetic random access memory for concentrating a magnetic field to the memory cell during write operation, and a method of manufacturing the same.
In order to achieve an aspect of the present invention, a method of manufacturing a magnetic random access memory is composed of:
forming a first magnetic film over a substrate,
forming a tunnel insulating film on the first magnetic film such that the tunnel insulating film has a curvature,
forming a second magnetic film on the tunnel insulating film, and
etching the first magnetic film, the tunnel insulating film and the second magnetic film to form a memory cell. The etching is executed such that the curvature is excluded from the memory cell.
In order to achieve another aspect of the present invention, a method of manufacturing a magnetic random access memory is composed of:
forming a step-structured member over a substrate, wherein the step-structured member has first and second surfaces substantially parallel to a substrate surface of the substrate, a first distance between the first surface and the substrate surface being different from a second distance between the second surface and the substrate surface;
forming a first magnetic film on the step structure;
forming a tunnel insulating film on the first magnetic film such that the tunnel insulating film has a curvature; and
etching a portion of the tunnel insulating film to form a tunneling barrier layer, wherein the whole of the tunneling barrier layer is located over the first surface.
The first distance is preferably larger than the second distance.
The step-structured member preferably has a third surface which bridges the first and second surfaces, the third surface being substantially perpendicular to the first and second surfaces.
In order to achieve still another aspect of the present invention, a method of manufacturing a magnetic random access memory comprising:
forming a conductive portion on a substrate, the conductive portion having a conductive portion surface substantially parallel to a substrate surface at a first distance from the substrate;
forming an insulating portion on the substrate wherein the insulating portion has a insulating portion surface substantially parallel to the substrate at a second distance from the substrate, the first and distances being different from each other;
forming a first magnetic film on the conductive and insulating portions;
forming a tunnel insulating film on the first magnetic film;
forming a second magnetic film on the tunnel insulating film; and
etching a portion of the tunnel insulating film to form a tunneling barrier layer wherein the whole of the tunneling barrier layer is located over the conductive portion.
The formation of the insulating portion is preferably executed by the steps of:
forming an insulating film covering the conductive portion;
removing a surface portion of the insulating film to flatten the insulating film; and
etching back another portion of the flattened insulating film to form the insulating portion.
The method is preferably further composed of:
forming a magnetic portion between the conductive portion and the substrate.
In order to achieve still another aspect of the present invention, a method of manufacturing a magnetic random access memory is composed of:
forming a step-forming portion over a substrate;
forming a lower electrode to cover the step-forming portion and the substrate such that the lower electrode is protruded in a direction perpendicular to a substrate surface by the step-forming portion;
forming a first magnetic film on an electrode surface of the lower electrode;
forming a tunnel insulating film on the first magnetic film;
forming a second magnetic film on the tunnel insulating film; and
etching a portion of the tunnel insulating film to form a tunneling barrier layer. The whole of the tunneling barrier layer is located over the step-forming portion.
The step-forming portion is preferably formed of a magnetic material.
In order to achieve still another aspect of the present invention, an MRAM is composed of a substrate, a step-structured member formed on the substrate, a first magnetic layer formed on the step-structured member, a tunneling barrier layer formed on the first magnetic layer, and a second magnetic layer formed on the step-structured member. The step-structured member has first and second surfaces substantially parallel to a substrate surface of the substrate. A first distance between the first surface and the substrate surface is different from a second distance between the second surface and the substrate surface. The whole of the tunneling barrier layer is located over the first surface.
In order to achieve still another aspect of the present invention, an MRAM is composed of a substrate having a substrate surface, a conductive portion formed on the substrate, an insulating portion formed on the substrate, a first magnetic layer formed on the conductive layer, a tunneling barrier layer formed on the first magnetic layer, and a second magnetic layer formed on the tunneling barrier layer. The conductive portion has a conductive portion surface substantially parallel to the substrate surface, and the insulating portion has an insulating portion surface substantially parallel to the substrate surface. A first distance between the conductive portion surface and the substrate surface is different from a second distance between the insulating portion surface and the substrate surface
The MRAM preferably further includes a magnetic portion between the conductive portion and the substrate.